1. Field of the Invention
The present invention relates to a developer-bearing member with a surface comprising minute dielectric portions and minute electroconductive portions which are mixedly distributed, the dielectric portions comprising an amino coupling agent. The present invention also relates to an image formation method of developing a latent electrostatic image to a visible image using the above developer-bearing member.
2. Discussion of Background
In conventional image formation apparatus, such as electrophotographic copying machines, printers and facsimile apparatus, in which latent electrostatic images are formed on a latent-electrostatic-image bearing member and developed to visible images by a developer, dry type development units using a powder-like developer are widely used.
As such powder-like developers, a two-component type developer comprising a toner and a carrier, and a one-component type developer comprising a toner without containing a carrier, are conventionally known.
A two-component type development method using the above two-component type developer is capable of yielding relatively stable, good recorded images, but has the shortcomings that the deterioration of the carrier is easily caused, and the mixing ratio of the toner and the carrier tends to change while in use, so that the maintenance of an apparatus using this method is complicated. Furthermore, the apparatus using the two-component type development method is relatively oversized.
From the above viewpoint, the primary focus of attention is on a one-component type development method using the one-component type developer, which does not have the above-mentioned shortcomings as in the two-component type development method.
There are two types of one-component type developers. One is of a type which consists of a toner, while the other is of a type which consists of a mixture of a toner and an auxiliary agent.
Furthermore, there are two types of toners. One is a magnetic toner which contains magnetic particles, and the other is a non-magnetic toner which does not contain magnetic particles.
Generally, the above-mentioned magnetic particles are not transparent. Therefore, it is extremely difficult to obtain clear color images including full-color images and multi-colored images by use of a magnetic toner. Therefore, it is preferable to employ a one-component type development method using a non-magnetic toner when colored images are to be obtained.
In a development unit using the one-component type development method, a one-component type developer is held on a developer-bearing member and transported into a development zone where a latent-electrostatic-image-bearing member and the developer-bearing member face each other, and the latent electrostatic images formed on the latent-electrostatic-image-bearing member are developed to visible images by the one-component type developer. In such a development unit, however, it is required that a large amount of a sufficiently charged toner be transported into the above-mentioned development zone and used for the development of the latent-electrostatic images in order to obtain visible images with high quality and a predetermined image density.
When a one-component type developer consisting of a magnetic toner is employed, the above requirement can be met relatively easily because the one-component type developer can be held on a developer-bearing member by us of the magnetic force of an inner magnet built therein. However, when a non-magnetic one-component type developer is employed, it is difficult to meet the above requirement because the developer cannot be magnetically held on the developer-bearing member.
Various proposals have been made as countermeasures for the above-mentioned problem. For example, in Japanese Laid-Open Patent Application 61-42672, there is proposed a method of developing latent electrostatic images into visible images by the following steps: bringing a reversely rotating sponge roller made of electroconductive polyurethane, into pressure contact with a development roller having a float electrode and a medium resistivity of 10.sup.9 to 10.sup.11 .OMEGA..multidot.cm, serving as a developer-bearing member; mechanically transporting a toner into a gap between the sponge roller and the development roller, and tribo-electrically charging the toner; holding the charged toner on the development roller; forming a toner layer on the development roller, with regulating the thickness thereof with a blade; and bringing the toner layer into contact with latent electrostatic images formed on a photoconductor.
In this method, however, the intensity of an electric field formed near the surface of the dielectric portions of the development roller cannot be sufficiently increased, so that it is difficult to hold a large amount of the toner on the surface of the development roller. Accordingly, the amount of the developer that can be transported into the development zone decreases in the course of the development process. As a result, visible images with high density cannot be obtained.
In addition to the above, there is known a development unit having a structure by which an electric field is applied between a development roller and a developer supply member in such a direction that a non-magnetic toner is electrostatically moved toward the development roller. This structure, however, is not capable of depositing a sufficient amount of the toner on the development roller for obtaining images with high quality and high density.
As such toner supply members, there are known an electroconductive foamed member with an electric resistivity of 10.sup.2 to 10.sup.6 .OMEGA..multidot.cm as disclosed in Japanese Laid-Open Patent Application 60-229067, an elastic member with a skin layer as disclosed in Japanese Laid-Open Patent Application 60-229060, and a fur brush as disclosed in Japanese Laid-Open Patent Application 61-42672.
Furthermore, as such development rollers, there are proposed a metallic development roller with an uneven surface as disclosed in Japanese Laid-Open Patent Application 60-53976, a development roller covered with an insulating overcoat layer as disclosed in Japanese Laid-Open Patent Application 55-46768, a development roller with an overcoat layer having a medium electric resistivity as disclosed in Japanese Laid-Open Patent Application 58-132768, and an electrode development roller with an insulating surface and an electroconductive surface as disclosed in Japanese Laid-Open Patent Application 53-36245.
In conventional development units using a non-magnetic one-component type developer, a toner is triboelectrically charged by the friction between the toner and a toner supply member, such as a sponge roller disclosed in Japanese Laid-Open Patent Application 60-229067, an elastic roller disclosed in Japanese Laid-Open Patent Application 62-229060, and a fur brush disclosed in Japanese Laid-Open Patent Application 61-52663, the toner is electrostatically deposited on the surface of a development roller by the friction between the toner and the development roller, a toner layer is regulated by a thickness-regulating member such as a blade, whereby latent electrostatic images formed on a photoconductor are developed to visible images by the toner. As the materials for the development roller for such conventional development units, for example, insulating materials, materials with a medium electric resistivity and layered materials are employed.
In the development methods disclosed in the above references, the toner is deposited on the development roller by the triboelectric charging between the toner supply member and the development roller. However, as the above triboelectric charging is performed between the toner-deposited toner supply member and the toner-deposited development roller, sufficient charging cannot be obtained. The result is that the deposition of the toner on the development roller becomes insufficient for obtaining toner images with high image density.
The optimum deposition amount of a non-magnetic one-component type toner and a charge quantity of the toner in a development method using a non-magnetic one-component type developer will now be explained.
For black and white copying, the electric charge quantity of the toner is important and preferably in the range of 10 to 20 .mu.C/g. When the charge quantity is less than the above range, toner deposition on the background tends to occur and the obtained images are poor in sharpness. Furthermore, it is necessary that the toner deposition on the development roller be in the range of 0.1 to 0.3 mg/cm.sup.2, and that the toner deposition on an image transfer sheet be in the range of 0.4 to 0.5 mg/cm.sup.2. This toner deposition on the image transfer sheet is attained by setting the rotation speed of the development roller at 3 to 4 times the speed of a photoconductor. When the rotation speed of the development roller is set in the above range, there is a problem that a developed solid toner image has a higher density in a rear end portion of the toner image than in the other portion. This phenomenon is referred to as "toner rear end shifting". In order to solve this problem, the rotation speed of the development roller has to be set as close as possible to that of the photoconductor. In order to obtain high quality images by this setting of the rotation speed of the development roller, the deposition amount of the toner on the development roller must be increased and the number of revolution must be decreased.
On the other hand, in the case of color toners, with respect to the color characteristics thereof, the colored degree is smaller than that of black toners. Furthermore, it is necessary that the toner be deposited on the development roller in an amount of 0.8 to 1.2 mg/cm.sup.2 in order to prevent "toner rear end shifting". It is required that the charge quantity of the toner be in the range of 5 to 20 .mu.C/g, preferably in the range of 10 to 15 .mu.C/g in order to obtain stable toner images. It is also required that a so-called filming phenomenon, in which the toner is deposited on the surface of the development roller, be prevented so that the deposition amount of the toner and the charge quantity can be stably obtained for a long period of time.
In order to solve these conventional problems, the inventors of the present invention previously proposed an image formation method in which a one-component type developer comprising a non-magnetic toner, when necessary with addition of auxiliary agents thereto, is supplied to the surface of a development roller which is rotatably driven to transport the developer into a development zone where a latent-electrostatic-image-bearing member is directed to the above-mentioned developer-bearing member, so that the latent electrostatic images formed on the latent-electrostatic-image-bearing member are developed to visible images, characterized in that numerous micro closed electric fields are formed near the surface of the developer-bearing member by selectively causing the surface of the developer-bearing member to support electric charges, the charged toner is attracted by these closed electric fields to deposit the developer on the surface of the developer-bearing member, thereby developing the latent electrostatic images to visible toner images.
This method has many advantages over the conventional methods, including the advantage that the intensity of the electric fields can be significantly increased in comparison with the case where the conventional methods are employed, since a number of micro closed electric fields are formed near the surface of the developer-bearing member, and therefore a large amount of sufficiently charged non-magnetic toner can be deposited on the developer-bearing member and transported into the development zone.
However the above image formation method using the developer-bearing member with numerous micro fields near the surface thereof has the shortcoming that the triboelectric charging properties are significantly changed when a variety of materials are used in the dielectric portions in order to obtain the other properties which are necessary for the developer-bearing member, such as toner release characteristics, low frictional properties, and abrasion resistance. Therefore, it is very difficult to meet the requirements for the deposition amount and the charge quantity of the toner, and the other properties of the toner.